Process for the manufacture of high-alpha-cellulose fiber



Ap 21, 1931 G. A: RICHTER ET AL 1,801,732

PROCESS FOR THE MANUFACTURE OF'HIGH ALPHA CELLULOSE FIBER Filed Dec. 1, 1925 UNITED STATES PATENT OFFICE GEORGE A. RICHTER AND MILTON O. SCHUR, OF BERLIN, NEW HAMPSHIRE, AS-

SIGNORS TO BROWN COMPANY, OI BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE PROCESS FOR THE MANUFACTURE OF HIGH-ALPHA-GELLULOSE FIBER v Application filed December The present invention has relation to the manufacture of high alpha-cellulose fiber, (i. e., pulp containing a high percentage of alpha or resistant cellulose) and has for its.

object to produce such fiber from unbleached pulp by a continuous, economical and eflicient process, and also to produce a product having certain optimum characteristics, Such high alpha-cellulose fiber may, as hereinafter described, be produced by refining or purifying a raw pulp such as sulphite matter and other coloring substances remaining after the sulphite treatment, all of which are undesirable constituents ifsulphite pulp is to be used for certain purposes. In order to produce a cellulose product which will contain substantially only alpha-cellulose fiber, it is necessary further to treat sulphite pulp to eliminate to a practicable extent the less-resistant beta and gamma celluloses, together with the residual or unreacted ligneous and other coloring substances contained therein. The removal of non-alpha cellulose constituents present in unbleached sulphite pulp may be partly accomplished by the use of an excessive amount of "bleach in the bleaching process, but this presents the 'ineluctable condition that the alpha-cellulose content of the pulp suffers a material decrease by conversion into the oXy-celluloses. Moreover, excessive bleach has the tendency to weaken the fiber strength of the pulp. In carrying out the process herein described, it is found that an alkaline'digesting liquor is effective in reacting with and dissolving the less-resistant celluloses, as Well as ligno- 1, 1925. Serial No. 72,522.

celluloses and other colorin matters from unbleached sulphite pulp. igh alpha-cellulose may thus be produced by an alkaline digestion of unbleached sulphite pulp, a

caustic soda or soda ash solution or mixed caustic soda and soda ash solution being especially adapted for such purpose of digestion. By digesting sulphite pulp in a liquor of the proper strength for a suflicient period of time, and then washing liquor from the pulp, a product is obtained which contains about 94% alpha or resistant cellulose and which has a high color after bleaching. The digestion maybe carried out under greater than atmospheric pressure at a high temperature in closed digesters, but in many cases it is more economical and expedient to digest mospheric pressure.

The alkaline digesting treatment may be 7 practised with advantage after a portion of the ligneous and other non-alphacellulose constituents contained in the sulphite pulp have been removed or rendered reactive, by first treating such chlorinating' liquor, such as a bleach or chlorine solution. Such pretreatment is especially desirable when the sulphite pulp is undercooked, since such undercooking leaves a higher percentage of ligneous and other, 0

coloring substances than ordinarily. The exidizing or chlorinating agent reacts with a portion of the impurities, forming soluble re-' action roducts or products which are easily remova tion. This, however, is not essentially a bleaching operation, terially or substantially bleached thereby.

Owing to its properties and to the par- 7 it has received, as .will hereinafter be described, high alphaticular treatment which cellulose produced by the practice of the invention and having certain characteristics may be employed instead of rag fiber in the manufacture of highbgrade bond and ledger papers, and is a suita e substitute for cotton fiber in the preparation of the cellulose nitrates (more often termed the nitro-celluloses) and other cellulose esters such as the cellulose acetates and the cellulose xanthates 9 out the spent 60- in open tanks underatpulp with an oxidizing or le by the subsequent alkaline diges- 35 since the pulp is not ma- 4 employed in the manufacture of artificial silk, or for other purposes where a substantially pure alpha-cellulose fiber is required.

This invention may best be understood from the following description, when taken in connection with the accompanying drawing, which is a more or less diagrammatic and conventional representation of some of the various instrumentalities employed in the process, and indicates the course or flow of the pulp and the various liquids.

In accordance with our invention, the sulphite pulp obtained by the digestion of wood chips, e. g., spruce chips, under the requisite time, temperature, and pressure conditions in accordance with modern practice, in a suitable acid sulphite li uor (as, for example, Ca (HSO or NaHS 3 of a strength of about 1% combined and 5% to 6% free S0 is washed, thoroughly screened, and made up to about a 2% consistency. The pulp is continuously fed from a source of supply into a raw stock chest 1, from which it is drawn at a constant rate through a pump 2 into a regulator box 3, where the stock is diluted to and couch rolls 27, and scraped therefrom by.

doctors 28 into a sluice 6. The thickened stock, which is of about a 14% consistency, is continuously delivered through the chute, or sluice 6, into a mixing tank 7 equipped with agitators 29.

The consistency of the stock in the mixing tank 7 is reduced by hot alkaline liquor (in this case a caustic soda solution) which is fed into the tank at a constant continuous rate, depending upon the rate at which the pulp is fed, from a heater tank 8 stationed above it. The pulp is thoroughly agitated and mixed With the hot alkaline liquor delivered thereto in suflicient amount to yield a 2% to 5% pulp suspension at a temperature of about 170- 200 F., the liquor of which contains 0.5% to 0.8% caustic soda. The dilute pulp suspen sion is continuously removed from the mixing tank 7 by a pump 9, at the same rate at which it is fed thereinto, to a second set of thickeners 10, similar to the first set 4:. These thickeners effect a concentration of the hot stock suspension to a consistency of about 9% to 12%,the thickened stock being delivered therefrom, together with the caustic soda cooking liquor associated therewith, into a cooking tank 11. The thickened stock contains about 5% to 8% caustic soda based on the Weight of bone-dry sulphite pulp.

The hot white alkaline water which is removed from the thin hot alkaline stock mix ture by the thickeners 10 is returned to the heater tank through the conduit 30, and is reemployed in the mixing tank 7 with other stock fed thereinto from the first set of thickeners 4. Shouldthe stock consistency obtained by the first thickening be equal to that obtained by the second thickening, there will be a liquor balance so that no fresh water will be required in the heater tank. In actual operation, however, the stock consistency on the first thickeners is maintained slightly in excess of that on the second thickeners, so that the addition of a certain amount of fresh water to a concentrated caustic soda solution fed at a constant rate into the heater tank is necessary in order to maintain the desired caustic soda concentration therein. This concentrated caustic soda solution contains an amount of caustic soda, which is required for the treatment of the pulp, and is fed continuously in measured and controllable amounts or doses from a doser tank 15 into the heater tank. This solution, which may be, as for example, of 50% strength, is supplied from a storage tank 12 through a circulating pump 13 into a smaller storage tank 14, above it, at a constant rate. Any strong solution tending to rise above a predetermined fixed level in the smaller st0rage tank 14 is returned by an overflow pipe 159 into the caustic solution 'pump inlet. Liquor is supplied by gravity from the tank 14 through a pipe 57 connected to the bottom of the tank to the doser 15 therebelow, preferably of the type described in application for Patent, Serial No. 44,227, filed July 17, 1925, by Milton Schur.

The head of liquor in the doser may vary within relatively wide limits without affecting the operation of the doser, and in order to maintain the head within these limits an excess volume of caustic solution is supplied into the doser reservoir from the tank 14. Any liquor tending to rise above the overflow inlet of pipe 151 passes out of the doser reservoir and joins the overflow from the caustic storage tank,the mixed overflow being returned through the pipe 159 into the inlet of the pump 13. The caustic soda liquor is delivered through a pipe line 31 from the doser into the heater tank at a rate depending upon the concentration of the caustic in which it is desired to digest the sulphite stock, and the rate of the pulp feed through the system. The make-up water required to dilute the solution to the predetermined concentration is introduced into the heater tank from a fresh water supply line 32,the volume introduced being controlled by a float device 16 which actuates a diaphragm valve 33 and thereby maintains the desired fixed level of solution in the heater tank. It is desirable to maintain a boiling condition in the heater tank, so as to keep the temperature of the pulp suspension in the caustic soda cooking liquor in the mixing tank high, in order to increase the capacity of the cooking tanks hereinafter described. To accomplish this, steam is blown or injected from the steam supply line 36 directly into the caustic soda liquor,the temperature of the heating tank. being controlled by a thermostatic control device 34:,

which regulates the amount of steam blown into the liquor. Such heating gives a substantially uniform temperature throughout the entire solution, since it does not depend upon an indirect method of heat'transfer such as would be obtained by steam coils.

The pulp suspension in the mixing tank 7 ordinarily would be below boiling point, because of the lower temperature of the pulp and water associated therewith, which is delivered thereinto from the first set of thickeners 4. However, the pulp suspension in the mixing tank may also be maintained uni-- formly throughout at or near-boiling temperature, by injecting steam from the steam line 36 directly into the stock suspension. The

, temperature ofthe mixture in the tank may be controlled by regulating the amount of steam blown into the tank from the steam line 36'by a suitable thermostatic control device 35. This would also tend to increase the capacity of the cooking tanks. I

It is quite obvious that the system hereinbefore outlined is intended to give a very thorough mixture of th causticsoda solution and the sulphite pulp, a well as to rapidly and uniformly heat the mixed mass.

' would be very diflicult to uniformly mix or heat a small quantity of caustic liquor with a relatively large mass of thick fibrous stock, By practicing the method described, the stock suspension delivered from the thickeners 4; into the mixing tank 7 allows intimate mixing with the relatively large amount of heated caustic soda liquor delivered from the heater tank 8. By concentrating the resultant dilute stock suspension in the alkaline liquor to the desired consistency, say 10% to 12%, a uniformly mixed and sufliciently preheated concentrated suspension for cooking is obtained. Such a uniformly mixed and preheated suspension must be delivered to the cooking tanks in order to prepare, a finished product of uniform and optimum quality.

The second set of thickeners delivers the The stock overtion of the raw stock. When the process is initially started it requires three to eight hours, depending upon the rate at, which it is fed from the thickener 10, before any stock.

is delivered into the washers 45 In other words,digestion of the stock consumes three to eight hours, depending upon the characteristics of the sulphite stock employed.

As hereinbefore mentioned, the stock en-' tering the first cooking tank 11 is at, or near, the boiling point of water. This temperature is maintained during digestion of the stock by injecting or blowmg live steam from the steam line 36 at suitable points into the cooking tanks. The amount of steam delivthermostatic control devices 46, 46. "However, it is not essential to the process that'the' 'ered into the tanks is regulated, as by'the solution in the cooking tanks be maintained at its boiling point. Lower temperatures. say 175 F., may be employed during digestion of the stock and may give equally'good results; but to secure these results, it takes a much longer time. T Since the rate of reaction between the alkaline solution and the nonalpha-cellulose contentof the pulp depends .to a certain extent upon the temperature and the concentration of alkaline digesting agent, *in order to increase the rate of reaction between the caustic soda solution and the pulp when lower temperatures are employed, the caustic soda concentration in the pulp suspension may be increased, and the excess caustic washed out with the reaction products, thereby maintaining the cooking tanks at their maximum capacity. This would, of necessity, involve an increased caustic soda consumption in the process, so 7 that it is not especially recommendable. In actual practice, it is preferable to keep close.

to the vboiling point, and thus operate 'at Y maximum capacity. About a 10% fresh stock suspension is actually maintained in the two-tank cooking system, but this may of course be increased to 14% to 18%, if desired. However, the thicker the suspension maintained in the cooking tanks, the more power is required to agitate the stock therein to secure best results.

As previously stated, it may be desirable to pretreat the sulphite pulp with a solution of an oxidizing or chlorinating agent prior to the alkaline treatment, especially when such pulp is of a refractory or under cooked nature, and thus has a higher nonalpha cellulose content than ordinarily. In such cases, the washed and screened pulp is treated, approximately at room temperature, at a stock density of about-5%, for about one-half to one hour, preferably in a solution containing either 1% to 2% chlorine, or 2% to 6% of 35% limeableach (CaCCl based on the dry weight of fiber. The chlorine or bleach reacts with the ligne- -ous and other coloring substance contained in the pulp, forming soluble reaction products or products which are removable by the subsequent alkaline digestion. The pretreated stock is then fed into the raw stock chest 1 and is caused to undergo the alkaline digestion or treatment hereinbefore described in connection with the usual sulphite stock.

The hot digested stock suspension withdrawn from the top of the second cooking tank is delivered to the washers 4:5 associated with a dark brown, spent digesting liquor, containing about 10% of the caustic soda contained in the liquor prior to digestion. The spent liquor is washed from the pulp before it is sent to the bleachery. This spent liquor contains not only residual unconsumed caustic soda, but also sodium carbonate and various organic substances removed from the original unbleached sulphite pulp. The available heat content of such spent liquor may be utilized and the sodium constituent thereof recovered as described in a companion application, Serial No. 26,361, filed by George A. Richter, April 28, 1925. The washed pulp is of light color, generally a little darker in color than ordinary unbleached sulphite pulp, contains about 94% alpha-cellulose, has an average fiber length of 0.9 to 1.1 mm, and a copper number of 1.4 to 2.0.

It may be desirable to add a small amount of bisulphite or sulphurous acid solution, to neutralize and thereby remove any last traces of alkali which the washers do not eliminate; the addition of weak acid being especially desirable if the washer system is inadequate, since in such a case there is apt to be an appreciable quantity of caustic soda present, which would otherwise cause an alkaline condition in the bleachery. If alkali is necessary in the bleaching operation, it is better to provide for such alkali purposely, as will subsequently be described, rather than to depend on the alkali present in an incompletely washed alpha pulp. In actual practice, the bisulphite or sulphurous acid may, or may not, be added, according to economic expediency.

The treated stock is thickened to a consistency of from 10% to 16%, and is delivered to the bleachery. Since the alkaline digestion has removed from the original fiber a certain amount of ligneous and other coloring matter, together with less-resistant beta and gamma celluloses, the removal of these bleachconsuming constituents allows the bleaching of the fiber with less hypochlorite or bleach than is ordinarily required. In actual practice, about 4.0% to 7.0% of 35% limebleach (CaOCI based on the dry weight of pulp, is ordinarily sufiicient. Precautions must be taken to maintain a temperature not to exceed 90 F. in the bleaching tanks, in order to preserve the maximum strength of the fiber. If desired, a certain small proportion of caustic soda liquor may be added to the bleach liquor and pulp within the bleaching tanks, to retard the bleaching and thereby eliminate the possibility of lowering the alpha-cellulose content or injuring the fiber strength of the stock due to too violent bleaching. This is especially desirable when the alpha-cellulose content of the unbleached alpha fiber is somewhat lower than usual. The bleaching period ranges from 8 to 20 hours, depending upon the characteristics of the original sulphite fiber undergoing treatment. The stock leaving the bleachery has a color of 95100 (based on a certain color scale used by the Brown Company) and has substantially the same characteristics as the stock leaving the cooking tanks.

The bleached stock, which is of good white color, may be improved by a superbleaching operation. The superbleaching process may be similar to the one described in Letters Patent N 0. 1,47 0,556, issued October 9, 1923. In carrying out this superbleaching process, the washed, bleached high alpha-cellulose fiber is pumped to a two-tank system, similar to the two-tank cooking system shown on the accompanying drawing. These tanks are preferably tile-lined, and equipped with agitators resistant to the action of chlorine solution. The stock is continuously fed through the superbleaching two-tank system after it has been mixed with an amount of chlorine solution to give a stock suspension of from 4% to 6%,the chlorine solution containing about 0.2% to 0.5% chlorine, based on the weight of bone-dry fiber. No eifort need be made to control the temperature in the superbleaching tanks, which may approximate the temperature of water avaliable at the time. If it is desired to increase the rate of superbleaching and thereby raise the capacity of a given superbleaching plant, the temperature may be increased to not higher than 90 F. The stock passes from the superbleaching system into a small anti-chlor tank, into which is deliveredcontinuously a small stream of'bisulphite liquor, which serves to react with, and to neutralize, any residual traces of chlorine or hypochlorous acid which may be present in the stock. From this tank, the stock is delivered to a series of pulp washers, where the reaction products formed in the anti-chlor tank'are removed. The superbleaching may, if desired, be carried out on stock which has not been washed after its passage through the main bleaching operation. This, however, is not an entirely satisfactory method of operation, inasmuch as it necessitates a corresponding higher chlorine usage, owing to the reaction of the chlorine with the products of reaction of the original bleach. The superbleaching may require one to two degree of refining to which it is desired -to' "subject it) of very short and broken fibers, as

well as fibrillae and specks which are not removed by screening or rifiling. The relatively small per cent of short fiber alphacellulose stock obtained by classification may be used with advantage in the preparation of cellulose derivatives. The pulp stock is then sent to a wet machine, and finally taken off in sheet form, ready for shipment. It normally contains about 94% alpha-cellulose, has a color of 107-110, an average fiber length of 1.0-1.5 mm., and a copper number of 1.4-2.0. Under certain conditions it ma to convert only the long fiber fraction into high alpha-cellulose pulp by the alkaline digestion. In such event, the final bleached high alpha-cellulose stock may or may not be classified according to economical expediency.

The classifying of the pulp may be carried out in apparatus of the type disclosed and claimed-in application SerialNo. 755,730, '2; filed December 13, 1924, by George A. Richter and Elwood Ebie; or, if desired, the apparatus disclosed and claimed in application Serial No. 206,477, filed July 18, 1927, by

Robert A. Webber, may be used, in which case the long fiber portion of the pulp is selected from the short fiber portion, which contains not only the shortand broken fibers of the original pulp, but also most'of the impurities, including specks, resins, and other coloring material whichis apparently selectively adsorbed by this portion.

A stock thus produced from unbleached sulphite pulp is availablefor use in the production of cellulose esters and cellulose 'ethers, and in the manufacture of high grade papers where a substantially pure alpha-cellulose pulp is required.

One of the great advantages flowing from the practice of the herein described process is the high pulp yield obtained. As compared with the initial unbleached pulp, there is a yield of a pulp having the characteristics described which amounts to about 83 to 86 per cent thereof and about .38 to 43 per cent, based on dry wood, as compared with a yield of about 45% unbleached sulphite pulp,

based on dry wood. Another great advantage is that the process is a continuous one, beginning with t e original unbleached sulphite pulp and ending with the finished sheet a sodium base sulphite liquor. I

purified high alpha cellulose product con- 125 of alpha-cellulose stock. Moreover, the product-is remarkably uniform inv chemical and physical characteristics owing to the continuous, uniform treatment. A further advantage is that the process may be conducted at atmospheric pressure with relatively simple apparatus.

As previously indicated, while high alpha cellulose fiber may be produced by a caustic soda solution treatment of sulphite pulp formed either by the acid digestion of wood chips in a sodium bisulphite liquor or in a calcium bisulphite liquor, we preferably employ sulphite pulp resulting from the sodium base liquor, because of the more desirable physical characteristics in the final product. In this connection, it may be stated that while the high alpha cellulose fiber produced from the sulphite pulp liberated from wood chips by a sodium base acid liquor possesses an alpha cellulose content substantially the same as thatproduced from a calcium base acid liquor, its physicalcharacteristics, including y. strength, tear and folding endurance, are be desirable to classify the raw stock and then materially higher. This is undoubtedly due to the fact that the unbleached sulphite pulp resulting from a sodium base sulphite liquor liquor, resulting in a more uniformly cooked 100 pulp and necessitating lower maximum temperature for conversion of the wood chips to pulp. The superior hysical characteristics of the sodium base ber carry through, after the alkaline treatment test is increased relatively by the alkaline treatment to a much greater extentthan the increase in the tear test of a pulp produced by an acid calcium bisulphite liquor. In 110 certain cases, moreover, where an optimum product is aimed at, it may be desirable to carry out the bleaching operation in a chlorine solution or in a solution of soluble hypochlorite, place of the usual lime bleach, which'latter may increase the ash content of the fiber either by the formation of insoluble calcium compounds which are retained by the fiber, or by the adsorption by the fiber insoluble calcium compounds. Such procedure may be followed, especially when the initial raw stock is produced by employing tains very little inorganic matter, 'i. e., has i a very low ash content, since all the treat- ,mentshave been eifectedwith the use ofreagents capable of forming only'soluble prod ucts. Such a product is available for esteri to the finished high alpha cellulose stock. ln fact, its tear such as sodium hypochlorite, in

of lime or fication, where a low inorganic or ash content is especially important in the cellulosic material to be utilized.

While we have specifically described our process in connection with raw sulphite pulp, yet in some cases certain steps or features of our process may be practiced in the treatment of pulp derived from an alkaline digestion of raw cellulosic material. Again, while we have stated that certain temperatures are desirable in securing specified results, yet these temperatures may be varied without affecting other steps of the process as a whole. Thus, while the alkaline purification treatment is described as being carried on at an elevated temperature, yet when considered in connection with preceding and succeeding steps of our process, we may, by theutilization of a desired strength of solution and a desired proportion of alkaline compounds to fiber, carry on the alkaline treatment at materially lower temperatures, as stated in patent application, Serial No. 52,232, filed August 24, 1925, by George A. Richter. Moreover, the caustic soda may be replaced in part or in entirety by other alkaline compounds, such as sodium sulphide, the concentration of such other alkaline compounds being chosen to produce a product equivalent to that obtained by the use of caustic soda alone. We do not claim herein specifically the process claimed in said last-mentioned application, nor do we claim herein generically the process of producing a high alpha cellulose wood fiber claimed in our application, Serial No. 716,154, filed May 27 1924.

What we claim is:

1. A process for the production of high alpha-cellulose fiber, which comprises the following steps: diluting pulp to a predetermined consistency, thickening said pulp, mixing said thickened pulp with an alkaline digesting liquor to secure a substantially uniform suspension of pulp in said liquor, thickening the uniform pulp suspension to a predetermined consistency, and digesting the uniform and thickened pulp suspension in said liquor.

2. A process for the production of a high alpha-cellulose fiber, which comprises thickening dilute pulp, mixing said thickened pulp with an alkaline digesting liquor to secure a substantially uniform dilute pulp suspension in said liquor, recovering and re-employing the white alkaline liquor removed during such thickening in mixing with other raw pulp, and digestingthe uniform and thickened pulp suspension in, said recoveredliquor.

3. A process for the manufacture of'a high alpha-cellulose fiber, which comprises forming a pulp suspension at about a 2% consistency, thickening said pulp to about a 9% to,12% consistency, mixing saidthickened pulp with an alkaline di esting liquor to secure about a 2% to 5% su stantially uniform pulp suspension-in said liquor, thickening the uniform suspension to about a 10%-12% consistency, and digesting the uniform and thickened pulp suspension in said alkaline liquor.

4. A process for the production of a high alphaecellulose fiber, which comprises the following steps intimately mixing stock with an alkaline digesting liquor, thickening the resulting suspension, and re-employing the white liquor removed during such thickening with other stock in the mixing step.

5. A process for the production of a high alpha-cellulose fiber, which comprises the following steps: intimately mixing pulp with an alkaline digesting liquor, thickening the resulting suspension, re-employing the white liquor removed during such thickening with other pulp in the mixing step after the addition of sufficient alkaline digesting agent thereto, and digesting said thickened pulp suspension in said liquor.

6. A process for the production of a high alpha-cellulose fiber, which comprises mixing and agitating pulp with a solution containing caustic soda, thickening the resulting pulp suspension to a predetermined consistency, re-employing the white alkaline liquor removed during such thickening in mixing and agitation with other pulp, and digesting said thickened pulp suspension in said liquor.

7. A process for the production of a high alpha cellulose fiber, which comprises continuously digesting a substantially uniform, continuously moving stream of pulp suspension in a parallel flowing alkaline digesting liquor, continuously washing the delivered stream of pulp free from spent digesting liquor, and continuously bleaching the stream of washed stock.

8. A process step in the preparation of high alpha cellulose fiber, which comprises continuously digesting a substantially uniform, continuously moving stream of pulp suspension in a parallel flowing alkaline digesting liquor.

9. A process for the production of high alpha cellulose fiber, which comprises continuously digesting a substantially uniform, continuously moving stream of pulp suspension in a parallel flowing alkaline digesting liquor, and continuously washing the delivered stream of pulp free from spent digestin liquor.

10. process for the production of high alpha-cellulose wood fiber, which comprises digesting a substantially uniform suspension of/wood pulp in an alkaline digesting liquor, washing the digested pulp, and bleaching the Washed pulp in a bleach liquor containing a relatively small proportion of alkali but sufficient to maintain a conditionof distinct alkalinity throughout the bleaching operation.

11. A continuous process for the produc- 5 sald pulp, and superbleaching said bleached pulp in a chlorine solution containing 0.2% to 0.5% chlorine based on the dry weight of pulp.

12. A process for the production of high alpha-cellulose fiber,-which comprises digesting a substantially uniform suspension of sulphite pulp in an alkaline digesting liquor,

Washing said digested pulp, bleaching said washed pulp "in lime bleaching liquor (CaOCI washing said bleached pulp, and superbleaching said pulp in a chlorine solution.

alphaecellulose fiber, which comprises digesting a substantially uniform suspension of pulp in an alkaline digesting liquor, bleaching said pulp in a calcium hypochlorite bleach, solution, washing said pulp free from bleaching constituent, superbleaching said pulp in a chlorine solution, and neutralizing and removing substantially all bleachin'g constituents from said superbleached ul p 14. A process for the manufacture of long fiber high alpha-cellulose stock, which comprises digesting a substantially uniform suspension of stock in an alkaline digesting liquor, washing said digested stock, subjecting said stock successively to bleaching and superbleachingoperations, and separating the long fiber stock from the short and broken fibers. i

15. A process for the production of long fiber high alpha-cellulose stock, which comprises digesting a suspension of pulp in an al kaline digesting liquor, washing said digested stock, bleaching the-washed stock, and separating the long fiber portion ,of the pulp from the short fiber portion.

16. A process for the manufacture of high alpha-celluiose fiber, which comprises digest ing pulp in an alkaline digesting liquor, washing the digested pulp, bleaching the pulp in calcium hypochlorite bleach at a temperature not exceeding 90 F.', wash1ng the bleached pulp, andsuperbleaching the washed bleached pulp in a chlorine solutio at a temperature not exceding 90 F.v

17. A process for the production of high alpha-cellulose fiber, which comprises thickening. dilute pulp, mixing said thickened pulp with an alkalinedigesting liquor to secure a substantially uniform suspension of pulp in said liquor, thickening the umform pulp suspension to a predetermlned consistency, and digesting the uniform and thickened pulp suspension in said liquor under I heat.

18. A process for the production of high '13. A process for the production of high.

ening dilute pulp, mixing said thickened pulp with an alkaline digesting liquor to secure a substantially uniform suspension of pulp in said liquor, thickening the uniform pulp suspension to a predetermined'consistency, digesting the'uniform and thickened pulp suspension inits liquor under heat, washing the digested pulp, bleaching the washed pulp in calcium hypochlorite bleaching liquor, bleached pulp in a chlorine solution.

19. A process which comprises treating sulphite pulp with a solution containing 1% to 2% chlorine based on the dry weight of fiber, washing said pulp free from said solution, and digesting said washed pulp at an elevated temperature and under atmospheric pressure at a consistency of about 10% to 12% andcontaining about 5% to 8% caustic soda based'on' stock. I

20. That step in the preparation of high alpha-cellulose fiber, which comprises treating a continuously moving stream of a sub stantially uniform and uniformly heated pulp suspension with an alkaline cooking liquor.

21. A continuous process for uniformly andthoroughly mixing pulp and a chemical,

which comprises diluting the pulp with an excess oftheichemical dissolved in a suitable liquid, thickening the mass to the desired consistency, and reusing the excess of the solution of the chemical after fortifying with more of the chemical.

22. A continuous process for rapidly. and

uniformly heating a fibrous mass, which comprises diluting the cold mass with hot water, thickening the mass to the desired consistency, and. reusing the elfluent hot water resulting after vreheating to the desired temperature.

23. A process for the production of a high alpha-cellulose fiber, which comprises the fol lowing steps: intimately mixing pulp with an alkaline digesting liquor, thickening the resulting suspension, re-employing the hot White liquor removed during such thickening with other pulp in the mixing step after the A addition of heat thereto, anddigesting said thickened pulp suspension in said liquor under heat.

24. A process for producing a purified sulphite fiber, which comprises cooking wood chips in a sodium bisulphite liquor, washing the resulting pulp, rifliing and screening said 0 Washed pulp to remove coarse mechanical impurities contained therein, digesting said pulp in an alkaline liquor, washing the digeslted pulp, and bleaching said washed Pu P- 25. A process for producing a purified sulphite fiber, which comprises cooking wood chips in a sodium bisulphite liquor, washing the resulting pulp, rifitling and screening and superbleaching said 75 the dry weight of sulphite 85 from said thickening step 105 65 alpha-cellulose fiber, which comprises thicksaid pulp to remove coarsevmechanical impurities contained therein, digesting saidand bleaching the redigested pulp inasodium pulp with an alkaline liquor to remove the base bleaching liquor.

non-alpha cellulose constituents contained therein, washing the digested pulp, and signatures.

bleaching in a sodium base bleaching liquor.

26. A process for producing a purified sulphite fiber, which comprises cooking wood chips in a sodium bisulphite liquor, washing the resulting pulp, rifiiing and screening such pulp to remove coarse mechanical impurities contained therein, digesting said pulp in an alkaline liquor to remove the non-resistant celluloses, lignin, and other undesirable matter contained therein, washing the digested pulp, bleaching the washed pulp in a sodium base bleaching liquor, and superbleaching the bleached pulp with a chlorine solution.

27. A process for producing a purified sulphite fiber, which comprises cooking wood chips in a sodium bisulphite liquor, washing the resulting pulp, rifiling and screening such pulp to remove coarse mechanical impurities contained therein, digesting said pulp in a solution containing caustic soda to remove the non-resistant celluloses, lignin, and other undesirable matter contained therein, washing the digested pulp, bleaching said washed pulp in a sodium base bleaching liquor, and superbleaching said bleached pulp with a chlorine solution.

28. A process for producing a purified sulphite fiber, which comprises cooking wood chips in a sodium bisulphite liquor, washing the resulting pulp, rifiling and screening such pulp to remove coarse mechanical impurities contained therein, digesting said pulp in an alkaline liquor to remove. the non-resistant celluloses, lignin, and other undesirable matter contained therein, washing the digested pulp, and bleaching said washed pulp in a liquor containing a bleaching agent incapable of forming insoluble compounds.

29. A process for the manufacture of a long fiber high alpha cellulose stock from screened unbleached pulp, which comprises initially selecting or separating the long fiber portion of the pulp from tha short fiber portion, and then digesting a substantially uniform suspension of the long fiber portion in an alkaline liquor.

30. A process for the manufacture of a long fiber high alpha cellulose fiber from unbleached pulp, which comprises separating the long fiber pulp from the short and broken fibers, digesting a substantially uniform suspension of the long fiber in an alkaline digesting liquor, Washing said digested stock, and subjecting said stock successively to bleaching and superbleaching operations.

31. A process for the production of a high alpha cellulose wood fiber, which comprises digesting wood chips in an acid sodium base cooking liquor, redigesting the resulting pulp in an alkaline sodium base cooking liquor,

In testimony whereof we have aflixed our GEORGE'A. RICHTER. MILTON O. SCHUR. 

